Propellent grains



July 14, 1964 K. E. RUMBEL ETAL PROPELLENT GRAINS 7 Sheets-Sheet 1 FiledJune 9, 1955 m w T m i w W/ m e WEMGO A g aw Eww w mwsw 550M M ME July14, 1964 K. E. RUMBEL ETAL 3,140,663

PROPELLENT GRAINS Filed June 9, 1955 7 Sheets-Sheet 2 wyz INVENTOR Kev/WE fi/MBEL,

MEL Vl/V GOA/5N, a 05557 6? Noam/r 4 BY Azcy CZ SCI/BLOCK AGENT July 14,1964 K. E. RUMBEL ETAL 3,140,663

PROPELLENT GRAINS Filed June 9, 1955 7 Sheets-Sheet 5 ALuM/Nz/M l V/EEINVENTORS KE/TH E P044554, MEL l///V COHEN, F0556;- 6? Ma /v7 ""4 BYflea/q 6. 50024006 W AGENT July 14, 1964 K. E. RUMBEL ETAL 3,140,663

PROPELLENT GRAINS Filed June 9, 1955 Sheets-Sheet 6 INVENTORS 5 IFUMEEL,

w/v CONE/V,

BY 15055 G Mam r 4 F 6. SZWFAOCK Wag/4w AGENT July 14, 1964 K. E. RUMBELETAL 3,140,653

PROPELLENT GRAINS Filed June 9,.1955 7 Sheets-Sheet '7 W k M INVENTORKEITH E. AuMsEL, MEL V/N COHE flask-er 6-. /VUG5N7' d I ARCH C. SewaoczAGENT 3,140,663 PROPELLENI GRAINS Keith E. Rumhel, Falls Church, Va,Melvin Cohen, Washington, D.C., Robert G. Nugent, Alexandria, Va., andArch (3. Scurloelr, Washington, BIL, assignors to Atlantic ResearchCorporation, Alexandria, Va, a cerporation of Virginia Filed June 9,1955, Ser. No. 514,254 15 (llaims. (Cl. MIL-98) This invention relatesto new and improved propellent grains having greatly increased eifectiveburning rates.

There is an ever-growing requirement, as for example, in the field ofrocketry, for the development of propellent grains which provideincreased propulsive performance. One way of accomplishing this is toincrease the loading density; that is, to fill a greater fraction of therocket motor chamber volume with the propellent grain. In so doing,however, an adequate rate of generation of pro pulsive gases must bemaintained. Although solid endburning grains are notable for their highloading density, their use in propulsive devices, as for example,solidpropellent rockets, has been limited by a low rate of generation ofpropulsive gases. The rate of generation of propulsive gases isproportional to the product of the propellent burning rate and theburning surface area. Although there are various expedients which can beemployed to increase the burning rate of the propellent material, thepropellent burning rates that have been obtained hitherto have not beensufiicient to permit the general use of solid end-burning propellentgrains.

Instead, it has generally been necessary to employ propellent grainshaving a burning surface area much greater than the grain cross sectionby resorting to such devices as extensive perforation of the propellentgrain, concentric, spaced tubular arrangement of the propellentmaterial, cruciform shapes and the like. Though providing the desiredlarge area of burning surface, these expedients possess the disadvantageof weakening the grain so that the solid'propellent material must meetstringent requirements as to strength and other physical properties,which impose rigid limitations as to the type of material which can beused. In many cases, also, such grains must be provided with specialexternal supporting and bracing structures.

Solid, end-burning grains, on the other hand, possess the strengthinherent in a structure which is solid throughout and can be supportedexternally by the walls of the chamber of use. As compared to perforatedgrains, operating temperature limits of solid end-burning grains arebroader, and propellent materials giving higher impulse can be employedwithout danger of weakening the physical structure of the grain.

Thus an increase in the effective or mass burning rate of solidend-burning grains which is sufficiently high to bring the rate of gasevolution within the desired range makes possible the use of suchgrains, with their attendant advantages, for many applications wherethey could hitherto not have been considered. Furthermore, the use ofsuch rapid-burning propellents, combined with other expedients forincreasing burning surface, such as perforations, provides aconsiderably higher rate of gas evolution than could hitherto beachieved.

The object of this invention is to provide propellent grains havinggreatly increased effective burning rates.

3,l i,663 Patented July 14, 1964 Other objects and advantages willbecome obvious from the following detailed description.

In the drawings:

FIGURE 1 comprises a series of duplications of high speed motion pictureframes.

FIGURE 2 is a sectional perspective of a solid, endburning grain showinga random dispersion of short lengths of wire.

FIGURE 3 is a sectional perspective of a solid, endburning grain showing.a dispersion of short lengths of wire which are longitudinallyoriented.

FIGURE 4- is a sectional perspective showing a solid, end-burning grainwith a single continuous wire.

FIGURE 5 is a transverse cross-sectional view taken along line 5-5 ofFIGURE 4.

FIGURE 6 is a sectional perspective showing a solid, end-burning grainwith a plurality of continuous wires.

FIGURE 7 is a plan view of the grain of FIGURE 6.

FIGURES 8 and 9 are sectional perspective views of other embodiments ofour invention.

FIGURE 10 is a plan view of a solid, end-burning grain with preshapedignition surface.

FIGURE 11 is a cross-sectional view taken along lines 1111 of FIGURE 10.

FIGURE 12 comprises duplications of high speed motion picture frames.

FIGURE 13 is a sectional perspective of another embodiment.

FIGURE 14 is a plan view of the grain of FIGURE 13.

FIGURE 15 is a plan view of a solid end-burning grain containing acontinuous, axially-embedded wire and continuous, concentric tubularmetal heat conductors and having a preshaped ignition surface.

FIGURE 16 is a cross-section taken along line l616 of FIGURE 15.

FIGURE 17 is a sectional perspective of a perforated grain with radiallydisposed continuous wires.

FIGURE 18 is a transverse cross-section taken along lines 18-48 ofFIGURE 17.

FIGURE 19 is a sectional perspective of a perforated grain withlongitudinally disposed continuous wires.

FIGURE 20 is a cross-section along line 2.02t) of FIGURE 19.

FIGURES 21 and 22 are sectional perspectives of still other embodimentsof our invention.

We have found that effective or mass burning rate can be greatlyincreased by embedding within the propellent grain a metallic heatconductor in the form, for example, of fine wire, filaments, strips andthe like, so that the entire surface of that portion of the metal whichlies within the body of the propellent grain is in intimate. contactwith the propellent matrix. The metal heat conductor may be dispersed inthe propellent matrix in the form of discontinuous short wires orfilaments or, preferably, in the form of a continuous wire or striporiented longitudinally in the desired direction of flame propagation.The increased burning rate of the propellent grain is due to the factthat the metal heat conductor, having a considerably higher thermaldiffusivity than the propellent material or its gaseous combustionproducts, effects rapid heat transfer from the high temperaturecombustion gases in the flame zone to unburned propellent within thegrain so that the flame propagates rapidly along. the metallic heatconductor. As a result, the rate of propagation of the burning surfacealong the metallic heat conductor is many times the normal propellantburning rate.

The heat conductor can be any metal having a substantially higherthermal diffusivity than the propellent material. It can be used in theform of wire of any cross-sectional shape, or thin strips which are fiator bent into shapes such as, for example, tubes, wedges and the like.The strips can be solid or perforated as, for example, in the form ofwire screening. The use of Wire is our preferred embodiment for thepractical reason of its more common availability. Although the followingdescription will be given in terms of the use of wire, it will beunderstood that similar results are obtained with metal heat conductorsof other shapes as aforedescribed such as thin strips, tubes, or thelike. The term wire as employed in this specification and claims refersto elongated metal filaments which are not necessarily circular in crosssection but which can also be of other cross-sectional shapes such asrectangular, oval or the like.

We have observed that when a metal wire is embedded in a solidpropellant, and the grain ignited, the propellant burns at its normalburning rate until a portion of the wire protrudes beyond the burningsurface into the hot combustion gases. The exposed wire is heated to ahigh temperature by the hot gases and this heat is then conducted by thewire into the unburned portion of the propellant. Burning then proceedsrapidly along the wire. The burning surface adjacent to the wirerecesses to form a cone with the wire at its apex. The recessingcontinues until an equilibrium point is reached Where the angle of thevertex of the flame zone at the wire, and thus the burning rate alongthe wire, remains substantially constant. Propagation of the burningsurface continues at a high rate along the wire. The rate of gasevolution is greatly increased by the large increase in burning surfaceproduced by recessing along the wire.

FIGURE 1 illustrates graphically the burning phenomenon which occurswhen a metal wire is embedded in solid propellant. The series shown areduplications of frames selected from a high speed motion picture of theactual burning of a propellent strand. A copper wire of 5 mil diameterwas embedded axially in a solid propellent strand which was 2 mm. thick,6 mm. wide and 40 mm. long. The propellant comprised a solid gelconsisting of polyvinyl chloride dissolved in plasticizer with a finelydivided oxidizer dispersed in the gel matrix. The plasticizer in thiscase was dibutyl sebacate and the oxidizer finely divided ammoniumperchlorate. All surfaces except the end burning surface were inhibited.The embedded wire terminated a short distance from the uninhibited endburning surface. The propellant was burned in a nitrogen atmosphere at1015 p.s.i.

Elapsed time, with the first frame A at time zero, is indicated at thebottom of each frame. In the first two frames A and B, at zero and 0.035second elapsed time, the wire is completely below the burning surfaceand the burning surface is plane. In frame C, at time 0.153 second, thewire projects into the flame zone approximately 0.05 inch and theburning surface is just starting to propagate along the wire with therecessing of the burning surface. Thereafter, as shown in frames DI, theburning surface propagates rapidly along the wire with continuedrecessing until the angle subtended by the equilibrium burning surfaceand the wire becomes established and remains constant. At this point theburning rate along the wire also becomes substantially constant. Therapid increase in burning rate along the wire is clearly shown by acomparison of the burning distances and elapsed time of 0.153 secondbetween frames A and C and the elapsed time of 0.136 second betweenframes C and I. The large increase in burning surface produced by therecessed cone can also be seen.

As aforementioned, before active propagation of the flame along the wirewill occur, a short length of the metal heat conductor must protrudeinto the burning TABLE I Silver Copper Aluminurn Wire diameter, inches0. 005 0.005 O. 005 Burning pressure, p.s.i 1, 015 1, 015 1,015 Exposurelength of Wire to lnltiate propagation of flame along Wire, inches 0.035 0. 047 0.052

For effective action, therefore, the wire must be of sufficient len thboth to provide for the initial exposure in the flame zone and forpropagation of the flame for some distance into the unburned propellantin which it is embedded. In general, we have found that the minimum wirelength required to achieve appreciable increase in effective burningrate is about 0.08 to 0.1 inch and, preferably, about 0.2 inch.

Substantial increases in burning rate are obtained by dispersing shortlengths of Wire in the propellent matrix. Dispersion of the Wire can beaccomplished, for example, by mixing the short lengths of wire with thepropellent material prior to extrusion or casting. The wires inpropellent grains prepared in this manner generally assume a more orless random pattern as shown in FIGURE 2 where metal Wires 1 areembedded in propellent grain 2. It will be noted, as shown in thedrawing, that a large number of the randomly dispersed wires are at anangle substantially less than 180 relative to the plane of the initialignition surface. The burning surface regenerates along such angledwires to produce involution and increased burning surface area. Somewhatimproved results in terms of increased burning rate can be achieved byorienting the dispersed short wires in the direction of flamepropagation, namely substantially normal to the initial burning surface.Such a grain is shown in FIG- URE 3 where wires 1 are embedded inpropellent grain 2 having initial burning surface 3.

The Wires dispersed in the propellent matrix must be at least about 0.08in. long to provide sufiicient length for initial exposure into theflame zone and flame propagation along the wire, as aforedescribed.Wires of 0.2 in. length produce higher burning rates than 0.1 in.lengths of wire of the same diameter. Some additional improvement can beobtained by further increasing the length of the dispersed wires as, forexample, to about 0.5 inch or longer. To some extent wire lengths willbe determined by the size of the propellent grain. In the case of largegrains, for example, wires 2 inches long or longer can be incorporated.

The amount of discontinuous wire introduced into the propellent matrixis not critical, although this is one of the factors which determinesthe specific increase in burning rate obtained. In other words, even theaddition of a very small amount will effect some increase. In mostcases, it is desirable to add at least about 0.5% and, preferably, atleast about 1% by weight of the propellant to obtain appreciableresults. In general, the larger the quantity of wire of a given lengthadded, the higher will be the effective burning rate. However, since theaddition of the short wires involves the introduction of substantiallyinert material into the propellant, thereby decreasing thegas-generating potential, in practice the amount incorporated will becontrolled to a considerable extent by this factor. For this reason, itwill generally be undesirable to add more than about 5 to by weight ofthe propellant although, in some cases, larger amounts may be feasible.

Table II summarizes the results obtained by incorporation of shortlengths of copper wire into a propellant comprising polyvinyl chloride,dibutyl sebacate and ammonium perchlorate. In B, one percent of copperchromite was added to the propellent mix as a burning catalyst. Thegrains were solid, end-burning strands as shown in FIGURE 2.Measurements were taken at 1000 psi.

TABLE II Amount Length Diameter Burning Percent wire added, of wire, ofwire, rate, increase Pressure peent inch inch inJsec. in burnexponent ering rate None 0. 54 0. 45 1 0.2 0.003 1. 23 128 0. 45 2 0. 1 0.003 1.0391 0. 38 1 0.1 0.003 0. S25 53 07 45 None O. 82 0. 40 l 0.1 0. 003 1. 2249 O. 36 2 0. 1 0. 003 1. 42 73 0. 4O 2 0. 2 0. 003 1. 72 110 0. 40 2 0.l O. 005 l. 10 34 O. 41

Although substantial increases in effective burning rate can be achievedby the dispersion of discontinuous, short wires in the propellentmatrix, we have found that vastly im roved performance is obtained withthe use of continuous wire which is longitudinally disposed in thedesired direction of flame propagation. Increases in burning rate of thepropellent grain which are several-fold greater than that obtained withdispersed, discontinuous, short lengths of wire can be obtained in thisway despite the use of considerably smaller proportions of metal.Apparently the reason for the large disparity in performance stems fromthe fact that, in the case of the discontinuous wires, the flamepropagates rapidly along each short length but is slowed substantiallyto the normal burning rate of the propellent material when it mustbridge the gap between the end of one wire and an adjacent wire. With acontinuous wire the flame continues to propagate rapidly anduninterruptedly through the entire length of the desired burningdistance. Another important advantage of the continuous wire is that itrequires the introduction of a minimum amount of inert material,generally no more than a fraction of one percent by weight of thepropellent.

FIGURE 4 shows an end-burning grain It) containing continuous wire 11axially embedded in the grain. The wire, which is normal to the initialburning surface 12, is disposed longitudinally in the direction of flamepropagation as shown by the arrow and is continuous throughout thedistance of flame propagation, in this case the full length of thegrain. The surfaces of the grain other than the end burning surface 12may be inhibited in any desired fashion. FIGURE 5 is a crosssectionalview of the propellent grain shown in FIG- URE 4. The mode of burning ofsuch a grain is shown in FIGURE 1. If desired, end 16 of the grain canbe left uninhibited and burning instituted from both ends. The flamethen propagates along the wire from both ends with doubled rate of gasevolution.

As shown in FIGURE 1, the burning surface of the grain shown in FIGURE 4recesses as the flame propagates along the wire to form a cone with thewire at its apex. As the flame proceeds along the wire, the flaring endof the lengthening cone increases in width and encompasses more and moreof the cross sectional area of the grain. If the grain is sufficientlynarrow,

the cone will eventually encompass the entire width of the grain andrapid burning of all the propellent material will continue until theother end of the wire is reached at which point only a small peripheralportion of the propellent material adjacent the end of the wire remainsunburned.

In many cases, particularly where the propellent grain has a relativelylarge cross-sectional area, it is desirable to embed a plurality ofcontinuous wires at spaced intervals as shown in FIGURES 6 and 7. Forexample, if a grain which is short relative to its width contains only asingle wire such as shown in FIGURE 4, the peripheral portion ofunburned propellent remaining when burning has progressed the fulllength of the wire may be considerably larger than desirable. This canbe avoided by introducing a plurality of wires as shown in FIG- URES 6and 7.

It is frequently desirable to achieve equilibrium pressure, namely thepoint at which burning surface area and, consequently, rate of gasevolution, becomes substantially constant, as quickly as possible. Wehave found that establishment of equilibrium can be hastened in severalways.

The use of a plurality of wires as shown in FIG- URES 6 and 7 increasesgreatly the rapidity with which the equilibrium burning surface area canbe accomplished. In the case of a single wire, the burning surfacepresented by the recessing cone continues to increase in area until theflaring end intersects the peripheral edge of the grain or until burningreaches the end of the wire, as, for example, in the case of a grainwhich is short relative to its width. Rate of gate evolution continuesto increase until surface area of the cone becomes constant. Such highprogressivity can be advantageous for some applications but not whererapid establishment of a constant burning surface area is desirable.When a plurality of continuous wires is used, the recessed conesincident to each wire soon intersect at their flaring ends and from thispoint on, the burning surface area remains constant as the flameproceeds along the wires.

The equilibrium state can also be established more rapidly by exposureof the wires a short distance beyond the initial ignition surface. InFIGURE 4, the wire terminates at the initial burning surface 12. Uponignition, the grain will burn for a short distance at the normal rate ofthe propellent material itself until a short length of the wireprotrudes into the hot combustion gases. When the protruding end of thewire becomes sufliciently hot to initiate propagation of the flame alongthe wire, the effective or mass burning rate will increase rapidly untilan equilibrium maximum is reached. To initiate flame propagation alongthe wire more rapidly, the wires can be embedded in the grain in such away that the ends of the wire protrude from the ignition surface asshown in FIGURE 6 where wire ends 13 extend for a short distance beyondignition surface 12. In some cases such exposed wire ends may causepractical difliculties because they may be broken off during handling ofthe propellent grains. This can be obviated by indenting the ignitionsurface as for example in the form of cones 9 or other depressions intowhich the wire protrudes, as shown in FIGURES 8 and 9.

We have also found that recessing the ignition surface adjacent to thewires, preferably in the form of cones, with the wire exposed at theapex, as shown in FIGURES 8 and 9, hastens establishment of theequilibrium burning surface area. Any degree of preconing which bringsthe initial burning surface into a closer approximation of theequilibrium burning surface than an initial plane surface results inmore rapid establishment of equilibrium. Thus, equilibrium is morequickly reached by the grains shown in FIGURES 8 and 9 than by the planesurfaced grains shown in FIGURES 4 and 6.

Most rapid establishment. of equilibrium burning surface area isobtained by preconing the initial ignition surface so that it has ashaped area which closely approximates or is substantially the same asthe equilibrium burning surface area so that equilibrium is establishedalmost immediately after ignition. In such a grain design, the angle ofthe vertex of the recessed cones should closely approximate theequilibrium angle and the cones should intersect with each other and theperiphery of the grain at substantially the same points at which theywill intersect during burning in the equilibrium state. FIGURES 10 and11 illustrate an end burning propellent grain having the ignitionsurface 12 preconed in such a way that it has a shape and surface areawhich is substantially the same as the equilibrium burning surface asburning proceeds along the seven spaced wires 11. The cones 9, whichflare out from the wire exposed at the apex of each, intersect eachother and with the periphery of the'grain 15 to form inwardly curvedridges 24 and apical points 25.

The preshaping of the ignition surface to simulate the equilibriumburning surface of an end burning grain is determined by such factors asthe number and spacing of the continuous wires, the metal of which thewires are made, the thickness of the wire and the particular propellentmaterial. The cone angle, for example, varies with the thermaldiffusivity of the particular metal, as will be seen below. Thesefactors can readily be determined by those skilled in the art and theparticular grain ignition surface designed accordingly.

Table III shows the enormous increase in effective burning rate obtainedby embedding a continuous metal wire in the propellent grain. Thepropellents employed in these burning tests were end-burning grains withan axially embedded, continuous Wire normal to the initial burningsurface and longitudinally disposed in the direction of flamepropagation substantially as shown in FIGURE 4. The solid propellentmaterial comprised 12.44% polyvinyl chloride, 12.44% dibutyl sebacate,74.63% ammonium perchlorate, and 0.49% stabilizer. The wire in each casewas mils in diameter.

TABLE III Properties of the Metal Burning Ratio of Rate Burning WireAlong Rate Thermal Wire Along Diffusi- Melting (in/sec.) Wires to vityat Tempera- Standard 650 0. ture 0.)

Rate 1 (cm. /sec.)

1 Normal burning rate of the propellent material=0.50 in./sec.

2 A square filament cut from 0.005inch magnesium sheet was used. 3 MusicWire.

4 Est.

As shown in Table III, the increase in burning rate of the propellentvaries with the particular metal used as the heat conductor. Theproperties of the metal which are apparently involved in determining itsemcacy are its thermal diffusivity and its melting point. The higher thethermal diffusivity of the metal, the more rapidly it conducts the heatto the unburned portion of propellent and the more rapid is the burningrate along the wire. Silver, for example, which has a high thermaldiifusivity of 1.23 cm. /sec. at 650 C. effected an increase in burningrate of 430% whereas platinum with considerably lower thermaldiffusivity of 0.35 cmF/sec. at 650 C. increased the burning rate by190%. Higher melting points also increase eiiicacy of the metal as canbe seen by a comparison of copper and aluminum. Aluminum melts at a muchlower temperature than copper and despite a somewhat higher thermaldiffusivity increases burning rate along the wire to a considerablylesser degree. Similarly tungsten, which has about the same thermaldiffusivity as magnesum but a much higher melting point is corisiderablymore effective in increasing burning rate. Apparently the higher themelting temperature of the Wire, the longer is the length of the wirewhich projects into the flame zone, thereby providing a greater area forheat transport from the hot gases to the wire.

Decreasing rates of heat transfer by the metallic conductor result inincreasing cone angles at the apex. The larger the cone angle, theshallower is the cone and the less is the available burning surface areawith concomitant reduction in effective or mass burning rate. This isgraphically illustrated in FIGURE 12 which shows duplications ofphotographs taken during burning of propellent strands containingaxially embedded continuous wires of silver, aluminum, platinum andsteel.

Metal alloys can be employed advantageously in some cases, particularlywhere the alloying serves to increase melting point without adverselyaffecting thermal diffusivity to any substantial extent.

We have also found that the emcacy of metals such as silver and copper,which have high thermal diffusivity but relatively low melting points,can be enhanced appreciably by plating with a metal of high meltingpoint such as chromium, and the like. The high-melting metal provides ashell in which the lower-melting core, even though molten, is supportedto provide a continuous path of low thermal resistance from the flamezone to the propellent. Generally speaking, where the plating metal hasa substantially lower thermal diffusivity, the coating is desirablyrelatively thin, as for example in the order of up to about 0.001 inchand preferably less. Thick coatings may, otherwise, provide suflicientthermal resistance to radial heat transfer to counterbalance theadvantage gainedby raising the effective melting temperature of the heatconductor. We have obtained additional increases in eifective burningrate of 5% and more by plating silver and copper Wires with 000025 and0.0005 inch coatings of chromium. These results were obtained by burningstrands of the polyvinyl chloride propellent aforedescribed containingthe continuous plated wires axially embedded substantially as shown inFIGURE 1.

The thickness of the wire or other metal heat conductor is not criticalinasmuch as the increase in effective burning rate is due to the higherthermal diffusivity of the metal relative to the propellent material.The thickness of the metal conductor does, however, influence to somedegree the extent of burning rate increase. For example, the greatestincreases generally are obtained with wires having a thickness of about2 to 10 mils, although large increases are also obtained with boththinner and thicker wires. In the case of metal heat conductors which,in their cross-sectional dimensions are considerably Wider than they arethick, such as metal strips or tubes, it ap pears to be the smallerdimension, namely the thickness, which affects the degree of burningrate increase.

We have observed that at pressures of 600 p.s.i. and higher, thepressure exponent of the burning rate along the wire or other metal heatconductor is decreased by an increase in conductor thickness. Above acertain thickness, which varies with the particular metal and propellentmaterial, the pressure exponent becomes even less than that of thepropellent itself. Thus the use of continuous wires embedded in thepropellent grain affords a means not only of considerably increasing theeffective burning rate but alsoof simultaneously improving the pressureexponent.

Where improvement in pressure exponent is an important consideration,wires of greater thickness can be used although the considerableincreasein effective burning rate is somewhat less than the maximumobtainable. In certain applications, it may be desirable to obtainmaximum possible burning rate and the wire species and thickness can bechosen to effectuate this.

One of the practical considerations which may determine, to some extent,the thickness of the Wire or other heat conductor, is the undesirabilityof introducing such large amounts of inert material as substantially todecrease the gas-generating potential of the propellent. From this pointof view, a maximum heat conductor thickness of about 30 to 50 mills willprobably be desirable in most cases.

Table IV summarizes burning test results obtained respectively withcontinuous copper, silver, tungsten and molybdenum wires embedded inend-burning grains substantially as shown in FIGURE 4. The propellentmaterial comprised 12.44% polyvinyl chloride, 12.44% dibutyl sebacate,74.63% ammonium perchlorate, and 0.49% stabilizer. Measurements weremade at a pressure of 1000 p.s.i. The results show the large increase ineffective burning rate achieved by use of the continuous embedded wires,the effect of varying wire diameter and the improvement in pressureexponent with increasing wire diameter. For example, the 3-mil copperwire increased effective burning rate by 411% as compared with theburning rate of the propellent alone and also gave a considerableimprovement in pressure exponent. With the mil copper wire, effectiveburning rate increase, while not quite so large, was almost 3-fold andpressure exponent Was reduced from 0.43 to 0.20. The trends were similarfor the other metals tested. As was to be expected, maximum increases ineffective burning rate obtained with tungsten and molybdenum wereappreciably less than those obtained with copper and silver because ofthe considerably lower thermal diffusivities of the former two metals,although, to some extent their lower thermal diffusivity was offset bytheir high melting points.

TABLE IV COPPER Wire Burning Increase in diameter, rate, burningPressure inch in ./sec. rate, percent exponent None 0. 46 0. 43 0.0011.12 142 0.75 0.002 1. 80 291 0. 58 0.003 2. 35 411 0.35 0. 005 2. 3780. 31 0. 007 2. 10 357 0. 21 0. 010 1. 78 287 0. 20

SILVER None 0. 44 0. 45 0.001 0.81 84 0. 87 0.003 1. 80 309 0. 60 0. 0052. 20 400 0. 40 0. G07 1. 86 323 0.45 0.010 2.08 370 0.15

TUNGSTEN None 0. 46 0. 46 0.001 1. 10 140 0.75 0. 003 1. 63 254 0.400.005 1. 55 237 0.33

MOLYB DENUM None 0. 45 0. 43 0.003 1. 30 189 0.40 0.005 1. 50 234 0.320. 011 1. 28 184 0. 19

The embedded metal heat conductors are effective regardless of thespecific nature or composition of the propellent although the specificincrease in effective burning rate will vary to some extent according tothe specific propellent composition. They can be employed both withcomposite type propellents which comprise a fuel and external oxidizer,such as the polyvinyl chloride propellent previously described, Thiokol,polystyrene and polyester type propellents and the like, with single anddoublebase nitrocellulose propellents, pressed ammonium nitratepropellents etc. Table V illustrates the efficacy of a continuous S-milcopper wire in end-burning propellent grains of different composition.

APolyvinyl chloride 12.5%; dibutyl sebacate 12.5%; ammonium ntitgaire75% (3450/6900 r.p.m. grinds in ratio of 1:1); plus 0.5% added s a 1izer.

B-Nitracellu1ose (12.6% N) 52.1%; nitroglycerin 39.2%; diethyl phthalate6.6%; 2-nltrodiphenylamine 2.1%; Gandclilla wax 0.01%.

Example I A solid, cylindrical, end-burning propellent grain 1.9 inchesin diameter was cast containing 19 copper wires of 7 mil diameter. Thewires, which were spaced at intervals through the propellent matrix andpositioned normal to the initial burning surface, were longitudinallydisposed and continuous throughout the length of the grain. Thepropellent material was a solid plasticized polyvinyl chloride gelcomprising polyvinyl chloride, dibutyl sebacate, ammonium perchlorateand a stabilizer. A 9-inch section of the grain was static-fired atambient temperature and a pressure of 850 p.s.i. Effective burning ratewas 2.7 in./sec. as compared with the normal burning rate of thepropellent material itself of 0.68 in./sec. The increase in effectiveburning rate was of the order of 297%.

As aforementioned, the metal heat conductor, though conveniently used inthe form of wire, can also be employed in the form of continuous thinstrips which can be flat or bent into other desired shapes such as aV-shape or a tube. The effect on mass burning rate is substantiallysimilar to that obtained with wires. The burning surface along metalheat conductors which are substantially wider than they are thick,assumes the configuration of a V-shaped trough rather than the coneincident to a wire. As in the case of wires, a plurality of strips ortubes can be employed.

The various expedients for hastening the establishment of theequilibrium burning surface, discussed above in connection with the useof wires, can be employed with thin, wide conductors, such as protrusionfrom the igni tion surface, and pre-troughing the ignition surface adjacent the heat conductor.

FIGURES 13 and 14 show a solid, end-burning propellent grain containinga V-shaped metal heat conductor 18 which is disposed longitudinally thefull length of the grain with one end exposed at the ignition surface12.

FIGURES 15 and 16 show a concentric tubular arrangement of metal heatconductors 19 with a Wire 11 embedded axially. The ignition surface 12is preshaped to a configuration which closely approximates theequilibrium burning surface. The flaring ends of coned recess 17 withthe protruding central wire at its apex intersects with the circularV-shaped trough 20, which has the first concentric tube protruding atits apex, to form a ridge 21. The outer flaring edge of this trough inturn intersects with the second trough having the outer concentric metaltube at its apex to form a second ridge. The second trough flares out tointersect with the periphery of the grain at '22. The wire end 13 andtube ends 23 protrude from the ignition surface.

Example II A strip of copper 5 mils thick and 2 mm. Wide was bentlongitudinally into the shape of a V with an angle of 45, the flaringsides each being 1 mm. wide. The V-shaped strip was embeddedlongitudinally in a solid, end-burning polyvinylchloride propellentgrain substantially as shown faces except for the end ignition surface.

in FIGURES 13 and 14, which was inhibited on all sur The grain wasburned at 15 p.s.i. Burning rate along the V-shaped strip was 1.35 in./sec. as compared with a burning rate of 0.43 in./ sec. for thepropellent in the absence of the metal heat conductor. Increase in massburning rate was 214% The large increase in effective burning rate madepossible by the incorporation of metallic heat conductors into thepropellent matrix, particularly in the form of continuous wires orstrips which extend substantially the entire distance of flamepropagation, makes practical the use of solid, end-burning propellentgrains for many applica tions, as for example in rocketry, wherehitherto their use was impossible. This is of great importance becauseof their other advantages as compared with perforated grains, such ashigher loading density, and greater strength. Propellants of higherimpulse can be employed without danger of weakening the physicalstructure of the propellent grain and Wider operating temperatures canbe employed.

Although the preceding description has been in terms of solidend-burning grains because of the enormous improvement in burning rateand other properties, such as pressure exponent, of this type ofpropellent grain, our invention can also be applied very advantageouslyto other types of propellent grains, such as perforated grains. The

incorporation of metal wire into the matrix of a perforated grainresults in a propellent which burns with extreme rapidity by virtue ofthe combination of the increased effective burning rate along the metalwire and the large initial burning surface provided by the perforations.The wire may be continuous through the distance of flame propagation ormay be dispersed through the matrix in the form of short, discontinuouslengths of wire. As in the case of end-burning grains, the continuouswires provide a considerably higher effective burning rate than thediscontinuous wire.

The continuous wire can be positioned in the matrix of the perforatedgrain in a manner most suitable for the particular application. Forexample, in the grain shown in FIGURES 17 and 18, the embedded wiresradiate out from the central perforation 14 which provides the initialburning surface. With the exterior surface 15 inhibited, the flamerapidly propagates peripherally along the wires.

FIGURES 19 and 20 show an end-burning cylindrical grain with centralperforation 14 and a plurality of continuous wires which are normal tothe end-burning surfaces 12 and 15 and run the length of the grain. Ifboth the exterior surface 15 and the surface exposed by the centralperforation are inhibited, the flame propagates rapidly along the wiresfrom both ends of the grain. If the central perforation surface isuninhibited, the grain also burns outwardly from the central perforationbut propagation of this flame front is considerably slower because ofthe absence of wire in the direction of flame propagation. Such grainsare particularly suitable for some rocket applications since it makespossible venting of combustion gases produced at the end of the grainadjacent to the closed end of the rocket chamber through the centralperforation.

As in the case of solid grains, the heat conductor incorporated intoperforated grains may be in the form of wires or thin strips of metalshaped into any suitable configuration such as wedges, tubes, etc.

For many applications requiring the use of propellent grains, it isessential that a high burning rate be maintained throughout combustion.This requirement can be satisfied by extending the continuous heatconductor for substantially the entire distance of flame propagation ofthe grain, as shown, for example, in FIGURES 4, 6, 8, 9, 13, 17 and 19.There are some cases, however, where a very high impulse is required foronly a portion of the combustion cycle as, for example, until apropelled object is airborne, after which the rate of combustion gasproduction can be reduced. Such a requirement can be met by limiting thelength of the metal heat conductor so that it extends in the directionof flame propagation only as far as it is desired to obtain the highrate of burning conferred by the conductor. After burning has proceededalong the full length of the conductor, combustion of the grain thencontinues at the normal rate of the propellent grain material.

FIGURE 21 shows a solid, end-burning propellent grain in which the heatconducting metal wires 11, which are disposed longitudinally in thedirection of flame propagation with one end exposed at ignition surface12, do not extend. the full burning distance of the grain. Burningproceeds from the ignition surface at the high rate induced by theembedded heat conductors until the point at which they terminate withinthe grain, after which burning continues at the normal rate of the graincomposition until the full burning distance of the grain has beentraversed at end 16.

It will be understood that the various expedients aforediscussed whichcan be employed to regulate burning rate, pressure exponent,establishment of equilibrium pressure and the like, such as choice ofmetal species and thickness of the heat conductor, the use of one or aplurality of heat conductors, protrusion of the heat conductor from theignition surface, perforation, etc., can be employed both Where the heatconductor is continuous substantially throughout the entire burningdistance of the grain or Where it extends only for a predeterminedportion of the burning distance.

In certain applications, it may be desirable to employ a propellentwhich progresses from a relatively low initial impulse to a highimpulse. In such case, the metal heat conductor can be embedded in thegrain at a predetermined point spaced from the initial ignition surface.The spacing can be small or considerable depending on the particularsituation. An example of such a grain is illustrated in FIGURE 22 where12 is the initial ignition surface.

Although this invention has been described with reference toillustrative embodiments thereof, it will be apparent to those skilledin the art that it may be embodied in other forms Within the scope ofthe appended claims.

We claim:

1. A solid propellent grain, said grain comprising a selfoxidant, solidpropellent matrix, the combustion of which generates propellent gases,and having at least one initial, exposed ignition surface, said matrixcontaining embedded therein a plurality of elongated metal heatconductors substantially spaced from each other in the plane transverseto the direction of flame propagation, said metal heat conductors beingpositioned substantially normal to the plane of saidinitial ignitionsurface of said grain and being continuously and longitudinally disposedin the direction of flame propagation of the grain, said conductorswithin the body of the grain having a length of at least about 0.2 inchand having a maximum thickness of about 0.05 inch in at least onetransverse direction, the entire surface of said length of said metalconductors lying within the body of the propellent grain and being inintimate, gas-sealing contact with the propellent matrix, the burningsurface of said grain, after ignition, regenerating progressively alongeach of said metal heat conductors and in so doing, forming a recesswhich is substantially V-shaped in at least one plane with each of saidmetal heat conductors at the apex of said recess, thereby forming arecessed surface of substantially larger surface area than that of aplane burning surface, said metal heat conductors being spacedsufliciently apart to permit said recessing of the burning surface, themetal heat conductors thereby serving to increase the mass burning rate,and, thereby, the mass rate of gas generation of said propellent grain.

2. The propellant grain of claim 1 in which the metal heat conductorsare metal Wires.

3. The propellant grain of claim 1 in which the metal heat conductorshave a thin coating of a metal of higher melting point and the entiresurface of said metal-coated metal heat conductors lying within the bodyof said propellent grain is in intimate, gas-sealing contact with thepropellant matrix.

4. The propellant grain of claim 1 in which the ends of the heatconductors are exposed at the ignition surface of the grain.

5. The propellant grain of claim 4 in which the ends of the heatconductors are exposed at the apex of a recess in the ignition surface.

6. The propellant grain of claim 1 in which the metal heat conductorsare continuous substantially throughout the entire distance of flamepropagation of the propellant grain.

7. The propellant grain of claim 6 in which the metal heat conductorsare metal wires.

8. The propellant grain of claim 7 in which the metal wires are selectedfrom the group consisting of copper and silver.

9. A solid propellant grain, said self-oxidant, solid propellant matrix,the combustion of which generates propellant gases, and having at leastone initial, exposed ignition surface, said matrix containing embeddedtherein a plurality of elongated metal heat conductors substantiallyspaced from each other in the plane transverse to the direction of flamepropagation, said metal heat conductors being positioned substantiallynormal to the plane of said initial ignition surface of said grain andbeing continuously and longitudinally disposed in the direction of flamepropagation of the grain substantially throughout the entire distance offlame propagation, the ends of said heat conductors being exposed in theignition surface of said grain, said conductors within the body of thegrain having a length of at least about 0.2 inch and having a maximumthickness of about 0.05 inch in at least one transverse direction, theentire surface of said length of said metal conductors lying within thebody of the propellant grain and being in intimate gas-sealing contactwith the propellant matrix, the burning surface of said grain, afterignition, regenerating progressively along each of said metal heatconductors and in so doing, forming a recess which is substantiallyV-shaped in at least one plane with each of said metal heat conductorsat the apex of said recess, thereby forming a recessed surface ofsubstantially larger surface area than that of a plane burning surface,said metal heat conductors being spaced sufficiently apart to permitsaid recessing of the burning surface, the metal heat cohductors therebyserving to increase the mass burning rate, and thereby, the mass rate ofgas generation of said propellant grain.

10. The propellant grain of claim 9 in which the ends of the heatconductors are exposed at the apex of a recess in the ignition surface.

11. The propellant grain of claim 10 in which the metal heat conductorsare metal wire.

12. The propellant grain of claim 11 in which the metal heat conductorsare selected from the group consisting of copper and silver.

13. A solid propellant grain, said grain comprising a self-oxidant,solid propellant matrix, the combination of which generates propellantgases and having at least one grain comprising a initial, exposedignition surface, said matrix containing embedded and randomly dispersedtherein a plurality of spaced elongated metal wires having a minimumlength of about 0.08 inch and a maximum diameter of about 0.05 inch, theentire surface of said length of said metal wires lying within the bodyof the propellant grain and being in intimate, gas-sealing contact withthe propellant matrix, a substantial number of said randomly dispersedwires being at an angle, relative to the plane of said initial ignitionsurface, which is substantially less than the burning surface of saidgrain, after ignition, regenerating progressively along each of saidmetal wires positioned at said angle substantially less than 180, and,in so doing, forming a recess which is substantially V- shaped in atleast one plane with a wire at the apex of each formed recess, therebyforming a recessed surface of substantially larger surface area thanthat of a plane burning surface, said metal wires being spacedsufliciently apart to permit said recessing of the burning surface, themetal wires thereby serving to increase the mass burning rate and,thereby, the mass rate of gas generation of said propellant grain.

14. A rocket grain comprising propellant material, said grain having anexposed burning surface whereupon the principal ignition of said graintakes place, and a plurality of straight metal wires with diameters inthe range of 0.001 to 0.05 inch spatially and completely embeddedthroughout said propellant material and contiguous therewith, said wiresbeing oriented in a direction generally normal to all of said burningsurface and arranged and adapted to increase the burning rate of saidpropellant material and increase the structural strength thereof.

15. The rocket grain comprising a cylindrical solid self-combustiblepropellant material of the composite type comprising a major amount of asolid inorganic oxidizing salt and a minor amount of a rubbery binder,said grain having at least one of its ends exposed to form a burningsurface whereupon the principal ignition of said grain takes place, anda plurality of straight metal wires with diameters in the range of 0.001to 0.05 inch spatially and completely embedded throughout saidpropellant material and contiguous therewith, said wires being orientedin a direction normal to all of said burning surface and arranged andadapted to increase the burning rate of said propellant material andincrease the structural strength thereof.

solid self-combustible References Cited in the file of this patentUNITED STATES PATENTS

1. A SOLID PROPELLANT GRAIN, SAID GRAIN COMPRISING A SELFOXIDANT, SOLIDPROPELLENT MATRIX, THE COMBUSTION OF WHICH GENERATES PROPELLENT GASES,AND HAVING AT LEAST ONE INITIAL, EXPOSED IGNITION SURFACE, SAID MATRIXCONTAINING EMBEDDED THEREIN A PLURALITY OF ELONGATED METAL HEATCONDUCTORS SUBSTANTIALLY SPACED FROM EACH OTHER IN THE PLANE TRANSVERSETO THE DIRECTION OF FLAME PROPAGATION, SAID METAL HEAT CONDUCTORS BEINGPOSITIONED SUBSTANTIALLY NORMAL TO THE PLANE OF SAID INITIAL IGNITIONSURFACE OF SAID GRAIN AND BEING CONTINUOUSLY AND LONGITUDINALLY DISPOSEDIN THE DIRECTION OF FLAME PROPAGATION OF THE GRAIN, SAID CONDUCTORSWITHIN THE BODY OF THE GRAIN HAVING A LENGTH OF AT LEAST ABOUT 0.2 INCHAND HAVING A MAXIMUM THICKNESS OF ABOUT 0.05 INCH IN AT LEAST ONETRANSVERSE DIRECTION, THE ENTIRE SURFACE OF SAID LENGTH OF SAID METALCONDUCTORS LYING WITHIN THE BODY OF THE PROPELLENT GRAIN AND BEING ININTIMATE, GAS-SEALING CONTACT WITH THE PROPELLENT MATRIX, THE BURNINGSURFACE OF SAID GRAIN, AFTER IGNITION, REGENERATING PROGRESSIVELY ALONGEACH OF SAID METAL HEAT CONDUCTORS AND IN SO DOING, FORMING A RECESSWHICH IS SUBSTANTIALLY V-SHAPED IN AT LEAST ONE PLANE WITH EACH OF SAIDMETAL HEAT CONDUCTORS AT THE APEX OF SAID ACCESS, THEREBY FORMING ARECESSED SURFACE OF SUBSTANTIALLY LARGER SURFACE AREA THAN THAT OF APLANE BURNING SURFACE, SAID METAL HEAT CONDUCTORS BEING SPACEDSUFFICIENTLY APART TO PERMIT SAID RECESSING OF THE BURNING SURFACE, THEMETAL HEAT CONDUCTORS THEREBY SERVING TO INCREASE THE MASS BURNING RATE,AND, THEREBY, THE MASS RATE OF GAS GENERATION OF SAID PROPELLENT GRAIN.